1. Technical Field
Methods for forming capacitors of semiconductor devices are disclosed, and more specifically, methods are disclosed for forming capacitors having a stacked structure of metal layer-insulating film-metal layer and having its storage electrode formed of ruthenium (hereinafter, referred to as xe2x80x98Ruxe2x80x99) and dielectric layer formed of tantalum oxide (Ta2O5) film, which provides improved formation of dense Ru film using a CVD method at high temperature, thereby improving electrical characteristics of the capacitor.
2. Description of the Related Art
Currently, due to high integration of semiconductor devices, the cell size becomes smaller and the surface area of storage electrode is decreased. However, the capacitance of semiconductor devices is proportional to the surface area of storage electrode and it became difficult to obtain capacitance of capacitor sufficient for stable operation of devices.
Specifically, in case of high integration of DRAM device having a unit cell consisting of a MOS transistor and a capacitor, it is important to decrease the size of cell and increase the capacitance of capacitor.
The capacitance of a capacitor follows the equation of (Eoxc3x97Erxc3x97A)/T (Eo: permitivity of vacuum, Er: dielectric constant of dielectric film, A: surface area of capacitor, T: thickness of dielectric film). Since the capacitance of a capacitor is proportional to the area of a capacitor as in the equation, in the conventional capacitor, a dielectric film is a tantalum oxide film and a lower electrode is a polysilicon (poly-Si) layer treated with RTN (rapid thermal processing).
However, when a poly-si layer is used as the lower electrode and a Ta2O5 film is stacked on the lower electrode, oxygen atoms activated in the subsequent thermal process under oxygen gas atmosphere penetrate the Ta2O5 film to react with the poly-Si film, which is the lower electrode, thereby forming SiO2. As a result, the thickness of the Ta2O5 film is increased, the thickness Tox of effective oxide film cannot be reduced. This makes it impossible to form an effective oxide film consisting of tantalum oxide having a thickness of about 30 xc3x85 or less. And limits the formation of a capacitor with a storage electrode using the same.
In order to solve this problem, the lower electrode formed of metal films such as Ru and TiN is suggested to reduce the thickness Tox of oxide film. Since, the metal films have oxidation tolerance compared to poly-Si film, the metal films are effective in reducing thee thickness Tox of effective oxide film.
It is known that when Ru film is used as the lower electrode and the Ta2O5 film is stacked on the Ru film, dielectric constant is four times larger than when poly-Si is used as the lower electrode even if the structure of the Ta2O5 film is changed.
A conventional method for forming a capacitor of a semiconductor device using metal film as lower electrode is described.
A lower insulating layer having a planarized top surface is formed on a semiconductor substrate. Here, the lower insulating layer comprises a device isolation film, a word line and a bit line.
A storage electrode contact hole exposing a predetermined region of the semiconductor substrate is then formed by etching the lower insulating layer.
The storage electrode contact hole is formed by etching the lower insulating layer via a photo-etching process using a storage electrode contact mask.
Next, a contact plug having a stacked structure of polysilicon film/diffusion barrier film is formed to fill the storage electrode contact hole.
A barrier metal layer is formed on a top portion of the contact plug using TiN film.
A Ru film which is a metal layer for storage electrode connected to the contact plug is formed on the entire surface of the resulting structure.
A CVD (chemical vapor deposition) method performed at a temperature of about 260xc2x0 C. is used to form the Ru film. RMS (root mean square) of the Ru film for storage electrode is 4.2 nm (see FIG. 1).
Here, concerning the atomic concentration according to sputter time of Ru film, when the Ru film is formed at low temperature such as above, sputter time, i.e. the time for analyzing thin films from the surface and to the bottom in AES analysis increases. As a result, the amount of oxygen also increases and the oxygen atoms penetrate into the Ru film. Due to the penetration of oxygen atoms, the surface of the Ru film becomes rough (see FIG. 2). The plane and cross-sectional view of the Ru film may be observed through SEM (see FIGS. 3a and 3b).
When an annealing process is performed on the resulting structure under nitrogen gas atmosphere at a temperature of 600xc2x0 C. for 60 seconds, RMS increases and impurities in the Ru film removed leaving empty spaces. In addition, agglomeration phenomenon, phenomenon of metals combine together like water drops due to surface tension is generated. AS a result, the surface becomes rough and a barrier metal layer is exposed.
After the annealing process, a tantalum oxide film is deposited on the Ru film. Then, a metal layer for plate electrode is formed on the Tantalum oxide film using a Ru film or a TiN film.
Here, the oxygen atoms included in the Ru film during the annealing process oxidize the interface of the barrier metal layer in the subsequent process of forming tantalum oxide films, thereby forming an oxide film, which results in degradation of electrical characteristics of capacitors of semiconductor devices and lift-off of Ru films.
As described above, the conventional method for forming a capacitor of a semiconductor device has a problem in degradation of electrical characteristics of devices due to oxygen contained in Ru films. Also, Ru films stacked at a temperature of 260xc2x0 C. have low quality of film, thereby generating an agglomeration phenomenon during the subsequent annealing process to expose barrier metal layers.
Accordingly, a method for forming a capacitor of a semiconductor device is disclosed wherein a Ru film is formed using a CVD method at high temperature, which provides capacitor with improved film quality and characteristics during the subsequent process.
A disclosed method for forming a capacitor of a semiconductor device comprises:
(a) forming a lower insulating film having a storage electrode contact plug on a semiconductor substrate;
(b) forming a contact plug filling the storage electrode contact hole;
(c) forming a storage electrode electrically connected to the contact plug, wherein the storage electrode is a Ru film formed using a CVD method performed at a temperature ranging from 300 to 400xc2x0 C.;
(d) subjecting the surface of the storage electrode to a RTP under nitrogen gas atmosphere;
(e) forming a dielectric film on the surface of Ru film;
(f) subjecting the dielectric film to a annealing process; and
(g) forming a plate electrode on the dielectric film.
In the process for forming Ru films according to this disclosure, the Ru film formation is performed in NH3 gas, O2 gas and Argon gas.
The Ru film is stacked at a high temperature ranging from about 300 to about 400xc2x0 C., preferably from 300 to 350xc2x0 C. to decrease RMS, which prevents the agglomeration phenomenon generated during the annealing process, improves quality of Ru film, minimizes oxygen content in the Ru films and inhibits formation of oxide film on the interface between Ru film and barrier metal layer, thereby improving the characteristics of the capacitors.